Personally, I don't see a reason why a 2015 release would be a problem for the box office of JL if they keep reasonably away from other big releases- I'm putting that at 2-3 weeks either side and more importantly make sure your release doesn't clash with with any of them a la Superman Returns. They may indeed take up WB's December spot post the Hobbit trilogy as I expect Marvel/ Disney to place theirs in the Summer.

Just in case you don't realise, 2013 has a spate of potential blockbusters lined up as well- IM3 should get close to a billion, so should Star Trek Into Darkness (awesome film, by the way). Then there is Lone Ranger, which if it's anything close to the fun of Pirates could run close to a billion too, though personally I'm highly doubtful. Pacific Rim is another movie set to take up the mantle of Robots from last years' Transformers and could go big at the BO. Of course, there's Desolation of Smaug that will probably cross a billion as well. Then there is a small matter of the first Superman movie in years which is highly anticipated and looks awesome anyway.

Of course, if they think 2015 is a hurried affair it's a different matter. Though, if they have indeed laid the groundwork- two and half years is quite some time.